This invention relates to a synthesis of sucrose fatty acid polyesters.
Fatty acid esters of sucrose were first prepared commercially for use in the food industry as nonionic surfactants, and the mono-, di- and tri- esters were found to be useful as emulsifiers (Osipow, L., et al., 48 Ind. Eng. Chem. 1459 (1956)). Whereas these lower esters of sucrose readily hydrolyze to form normal food components on digestion in mammals, it was discovered that sucrose and structurally-related polyols which had more than four esterified hydroxyl groups were less readily absorbed (Mattson, F. H., and Nolen, G. A., 102 J. Nutrition 1171 (1972)).
Sucrose polyesters which have at least four hydroxyl groups esterified with fatty acid residues per sucrose molecule have many of the physical properties of ordinary triglyceride fat. Since sucrose polyesters are comparatively less digested or absorbed and thus are relatively low in available calories, they are useful as low calorie replacements of edible fats and oils in food products (Hamm, D. J., 49 J. Food Sci. 419 (1984)). Consequently, in recent years the scientific and industrial community has focused attention on this class of compounds. Sucrose polyesters have been suggested for a variety of low calorie food compositions (e.g., U.S. Pat. Nos. 3,600,186, 4,446,165, and 4,461,782).
Further research on the dietary use of sucrose polyesters led to the finding that certain non-absorbable, non-digestible polyesters interfere with the body's absorption of cholesterol and thereby provide a potential means for treating hypercholesterolemia (U.S. Pat. No. 4,005,195). Pharmaceutical studies of sucrose polyesters were expanded. Sucrose polyesters have since been suggested for treating acute and chronic exposures to lipophilic toxins (U.S. Pat. No. 4,241,054), and for dissolving gallstones (U.S. Pat. No. 4,264,583).
A variety of methods have been developed to synthesize this class of dietary and pharmaceutical compounds. These methods followed classical esterification procedures similar to those outlined for fatty acid esters generally; sucrose octapalmitate and octastearate were prepared as early as 1921 (Markley, K. S., ed., Fatty Acids, Interscience Pub. Co., N.Y., 1961, vol 2, p. 849). The methods include the reaction of sucrose with acid anhydrides (U.S. Pat. Nos. 2,931,802, 3,057,743 and 3,096,324) or with acid halides (U.S. Pat. Nos. 2,853,485, 2,938,898, and 2,948,717), and the transesterification of fatty acid esters with sucrose in a solvent (U.S. Pat. Nos. 2,893,990 and 3,248,381). The reactions were plagued by solubility problems, since the properties of the sucrose are entirely different from fats or fatty acids (Osipow, L., et al. supra at 1459). The solvents employed, notably dimethylformamide, which was found to be particularly suitable, were expensive and toxic. The protocols devised to remove solvent after the reaction were possible in practice only with great difficulty (U.S. Pat. Nos. 3,378,542 and 4,611,055).
The so-called "transparent" or "micro-emulsion" process was suggested to overcome these drawbacks of the solvent system. In this method, a fatty acid ester is dispersed in a solution of a solvent such as propylene glycol or water with the aid of an emulsifier such as an alkali metal fatty acid soap to form a micro-emulsion, and the solvent is removed from the emulsion. The reaction is then carried out in the absence of solvent as if the reactants were miscible or if they were dissolved in a mutual solvent, and the reaction product does not contain any solvent (U.S. Pat. No. 3,480,616). However, removing the emulsification solvent while maintaining the micro-emulsion is difficult and the sucrose polyesters formed are contaminated with the soaps used as emulsifiers (U.S. Pat. No. 4,611,055).
A solvent-free synthetic technique was reported to overcome the problems of both the solvent and the micro-emulsion methods (U.S. Pat. No. 3,963,699). This synthesis is a transesterification between sucrose and fatty acid lower alkyl esters, which are simply heated together in an inert atmosphere. Since sucrose melts at about 185.degree. C. and starts to decompose after a few minutes at its melting point, the rate of sucrose degradation on heating must be retarded; for this an alkali-free soap is added (U.S. Pat No. 3,714,144). Subsequent patents disclose improvements in yield by suggesting catalysts and changes in reactant ratios (U.S. Pat. Nos. 4,517,360 and 4,518,772).
The solvent-free transesterification system has drawbacks, however. The unreacted alkali metal soaps used to keep the sugar from decomposing must be separated from the sucrose polyester product afterwards (U.S. Pat. No. 4,611,055). The product is further contaminated by sucrose decomposition products because sucrose is so thermally unstable that it is difficult to melt without some thermal cracking. Thus, sucrose polyesters made using this method are often described as colored. (See, for instance, Example I of U.S. Pat. No. 3,963,699, and Example 6 of U.S. Pat. No. 4,611,055, which yield, respectively, "light yellow" and "pale yellow" sucrose polyesters, and the examples in U. S. Pat. Nos. 4,517,360 and 4,518,772 disclose bleaching steps.)
To obtain a homogeneous melt, it is necessary to stir the system, and the reactants are viscous and tend to agglomerate (U.S. Pat. No. 3,251,827). The reactants have poor affinity for one another, and the sucrose fatty acid ester intermediates that form hydrolyze or saponify readily (U.S. Pat. No. 3,792,041). Furthermore, to obtain a high degree of transesterification, the molar ratio of fatty acid esters to sucrose must be in excess of a stoichiometric amount, and a reaction mixture containing such an excess of the fatty acid esters, which are less viscous than sucrose, is easily susceptible to phase separation, which adversely affects the reaction (U.S. Pat. No. 4,611,055).